Preparation and Characterization of Model Catalysts for the HCl Oxidation Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 208

adjacent O ot during the dissociative adsorption process, while Cl ot is bound
on top to 1f-cus Ru. The second HCl molecule dissociates, transferring its H
atoms to the O ot H species, thereby forming water O ot H 2 . The adsorption
energy of the second HCl molecule is even 175 kJ mol 1 . Alternatively, HCl
adsorption can form a second O ot H species so that a hydrogen transfer be-
tween neighboring O ot H species may form water. This process is slightly
activated by 30 kJ mol 1 . 82 In any case, the formed water molecule O ot H 2 is
bound by 120 kJ mol 1 to the surface so that it desorbs around 420 K from
the surface. 81
At elevated temperatures (about 600 K) the remaining surface Cl ot species
can recombine to form Cl 2 . This process is activated by 228 kJ mol 1 and
constitutes the elementary step with the highest activation barrier encountered
in this catalytic cycle. Accordingly the catalyst temperature must be chosen
higher than 600 K to be able to liberate the desired product Cl 2 . 33,100,103 Yet,
the association of surface chlorine does not present the rate determining step
under typical reaction conditions. The rate determining step represents rather
the dissociative adsorption of oxygen! The reason is that under typical reaction
conditions the surface is mostly covered by chlorine (which is substantially
stronger bound than oxygen), so that oxygen adsorption, which requires two
neighboring free 1f-cus Ru sites, is eciently blocked. 83,97
In the catalyzed HCl oxidation over RuO 2 (110), which runs at about 600 K,
the surface reactions are intimately coupled to gas phase via adsorption and
desorption. In Figure 8.4 this interplay is illustrated by the yellow clouds
around the model catalyst/catalytic cycle. The experimentally determined
desorption temperatures of water, oxygen, or HCl are 420 K, 400 K or 550 K,
respectively. Readsorption of water inhibits the reaction since water blocks
active 1f-cus Ru sites and water can transfer H to Cl ot which then desorbs in
the form of HCl. Readsorption of Cl 2 acts also inhibiting since the Cl 2 ad-
sorption blocks two active 1f-cus Ru sites for dissociative oxygen adsorption.
Quite in contrast, the reaction order of the HCl oxidation reaction is positive
with respect to O 2 as found in recent kinetic experiments on RuO 2 powder. 32
Only recently, 100 the first values for turn-over frequencies (TOF) have been
reported for the HCl oxidation reaction over RuO 2 (110) and RuO 2 (100) model
catalysts. Reactivity experiments in a batch reactor indicate that independ-
ent of the used surface orientation 0.6 Cl 2 molecules are produced
per second and active site at 650 K (i.e., TOF ¼ 0.6 s 1 ), when starting with a
reaction mixture of P(HCl) ¼ 2 mbar and P(O 2 ) ¼ 0.5 mbar. Therefore, the
HCl oxidation maybe considered as being not structure sensitive. A similar
result was reported for the CO oxidation. 29

d n 9 r 4 n g | 8

.

8.6 Synthesis of Metal Oxide Fibers via
Electrospinning
Electrospinning is a versatile method to prepare fibers with diameters in the
range of typically several hundred nanometers. Fundamental requirements